Vibrating meters, such as for example, vibrating densitometers and Coriolis flow meters are generally known and are used to measure mass flow and other information for materials within a conduit. The meter comprises a sensor assembly and an electronics portion. The material within the sensor assembly may be flowing or stationary. Each type of sensor may have unique characteristics, which a meter must account for in order to achieve optimum performance.
Exemplary Coriolis flow meters are disclosed in U.S. Pat. No. 4,109,524, U.S. Pat. No. 4,491,025, and Re. 31,450 all to J. E. Smith et al. These flow meters have one or more conduits of straight or curved configuration. Each conduit configuration in a Coriolis mass flow meter has a set of natural vibration modes, which may be of simple bending, torsional, or coupled type. Each conduit can be driven to oscillate at a preferred mode.
Material flows into the flow meter sensor assembly from a connected pipeline on the inlet side of the sensor, is directed through the conduit(s), and exits the sensor through the outlet side of the sensor. The natural vibration modes of the vibrating material filled system are defined in part by the combined mass of the conduits and the material flowing within the conduits.
When there is no flow through the sensor assembly, a driving force applied to the conduit(s) causes all points along the conduit(s) to oscillate with identical phase or small “zero offset,” which is a time delay measured at zero flow. As material begins to flow through the sensor assembly, Coriolis forces cause each point along the conduit(s) to have a different phase. For example, the phase at the inlet end of the sensor lags the phase at the centralized driver position, while the phase at the outlet leads the phase at the centralized driver position. Pick-off sensors on the conduit(s) produce sinusoidal signals representative of the motion of the conduit(s). Signals output from the pick-off sensors are processed to determine the phase difference between the pick-off sensors. The phase difference between the two or more pick-off sensors is proportional to the mass flow rate of material flowing through the conduit(s).
The mass flow rate of the material can be determined by multiplying the phase difference by a Flow Calibration Factor (FCF). Prior to installation of the sensor assembly of the flow meter into a pipeline, the FCF is determined by a calibration process. In the calibration process, a fluid is passed through the flow tube at a known flow rate and the relationship between the phase difference and the flow rate is calculated (i.e., the FCF). The flow meter subsequently determines a flow rate by multiplying the FCF by the phase difference of the pick-off sensors. In addition, other calibration factors can be taken into account in determining the flow rate.
Due, in part, to the high accuracy of vibrating meters, and Coriolis flow meters in particular, vibrating meters have seen success in a wide variety of industries. One industry that has faced increased demands for accuracy and repeatability in measurements is the oil and gas industry. With the increasing costs associated with oil and gas, custody transfer situations have demanded improvements in measuring the quantity of oil that is actually transferred. An example of a custody transfer situation is pipeline transfer of crude oil, or even lighter hydrocarbon fluids such as propane.
One problem faced during measurement in custody transfer situations, and measurement of light hydrocarbons in particular, is outgassing or flashing of the liquid. In outgassing, the gas is released from the liquid when the fluid pressure within the pipeline, or the vibrating meter, is less than the fluid's saturation pressure. The saturation pressure is typically defined as the pressure at which a substance changes phases from a liquid or solid to a gas at a given temperature, i.e., the vapor is in thermodynamic equilibrium with its condensed phase. Therefore, the saturation pressure may change depending on whether the fluid is a pure substance or a mixture of two or more substances based on the mole fraction weighted sum of the components' saturation pressures according to Raoult's Law. The saturation pressure is sometimes referred to as the vapor pressure or the bubble point. In the present description, the pressure at which a substance changes phases from a condensed form (liquid or solid) to a gas for a pure substance or a mixture at a given temperature is referred to as the saturation pressure. While maintaining a fluid above the saturation pressure may not be problematic in some pipeline systems, it is particularly problematic as the fluid flows through any type of sensor or meter that has a reduced cross-sectional area. Measurements of various flow characteristics become increasingly difficult with fluids at pressures below their saturation pressure. Furthermore, in some circumstances, the fluid may oscillate around the saturation pressure. For example, the fluid may be above the saturation pressure during one point of the day, i.e., when it is cool in the morning; however, during the afternoon as the temperature increases, the saturation pressure may be lower and consequently, the fluid may be flowing through the system at a pressure below the saturation pressure.
Consequently, there is a need in the art for a system that can adequately maintain a fluid flowing through a fluid flow system above the fluid's saturation pressure. The embodiments described below overcome this and other problems and an advance in the art is achieved. The embodiments disclosed in the description that follows utilize flow characteristics obtained from the vibrating meter in order to adequately adjust the flow so the fluid is maintained above the fluid's saturation pressure while flowing through the vibrating meter.